The present invention relates to a hybrid continuously variable transmission including a continuously variable transmission mechanism and a gearing mechanism combined with the continuously variable transmission mechanism, and more particularly to a downshift control involving a transition from a drive taken over by the continuously variable transmission mechanism to a drive taken over by the gearing mechanism.
U.S. Pat. No. 4,735,113 discloses a V-belt type continuously variable transmission including a driver pulley, a follower pulley, and a V-belt drivingly interconnecting these pulleys. U.S. Pat. No. 4,907,471 discloses a hybrid continuously variable transmission in which the above-mentioned continuously variable transmission mechanism is combined with a gearing mechanism such that the gearing mechanism takes over a drive to provide a reduction ratio between a transmission input shaft and a transmission output shaft for start-up operation. The setting is such that this reduction ratio provided by the gearing mechanism is larger than the maximum or largest reduction ratio provided by the continuously variable transmission mechanism. For high speed operation where a relatively small drive force is required, the continuously variable transmission mechanism takes over a drive between the transmission input shaft and the transmission output shaft owing to engagement of a power interruption device, such as a clutch.
More specifically, the known hybrid continuously variable transmission includes a first clutch, namely a drive reverse clutch or a low clutch, and a second clutch, namely a high clutch, are provided. When the low clutch is engaged with the high clutch disengaged, an engine power is delivered by the gearing mechanism, while when, with the drive reverse clutch kept engaged, the high clutch is engaged, the engine power is delivered by the V-belt type continuously variable transmission. A transition from a drive taken over by the gearing mechanism to a drive taken over by the V-belt continuously variable transmission mechanism is effected by engaging the high clutch. Although the high clutch is engaged with the drive reverse clutch kept engaged, a one-way clutch is released to interrupt delivery of power from the gearing mechanism to the output shaft. Thus, the transistion in drive is made smoothly owing to the action of the one-way clutch.
The V-belt type continuously variable transmission mechanism includes a driver pulley with a driver pulley cylinder chamber, a follower pulley with a follower pulley cylinder chamber, and a V-belt drivingly interconnecting the driver and follower pulleys. For controlling the continuously variable transmission, a hydraulic fluid pressure acting within the driver pulley cylinder chamber is varied by a shift control valve. The shift control valve includes a spool which is operated via a shift operating mechanism by a shift motor in the form of a stepper motor under the control of a microcomputer controlled unit. With this arrangement, the hydraulic fluid pressure within the driver pulley cylinder chamber assumes different values, each corresponding to one of different positions which can be taken by the stepper motor. This hydraulic fluid pressure variable by the stepper motor is admitted to the high clutch.
As mentioned above, the high clutch is activated by the same hydraulic fluid pressure acting within the driver pulley cylinder chamber and thus controlled by the stepper motor. Thus, even if a quick downshift involving a disengagement of the high clutch is required, it takes a considerable time until the high clutch is disengaged since the hydraulic fluid pressure acting within the driver pulley cylinder chamber does not drop quickly. If the hydraulic fluid pressure within the driver pulley cylinder chamber is subject to a rapid drop, the tension on the V-belt is lost, causing the V-belt to slip. In order to prevent such slip of the V-belt, a large amount of hydraulic fluid has to be supplied to the follower pulley cylinder chamber, immediately. However, a pump cannot afford to supply of such large amount of hydraulic fluid to the follower pulley.
An object of the present invention is to improve a hybrid continuously variable transmission such that a clutch which is responsible for a transition from a drive taken over by a continuously variable transmission mechanism to a drive taken over by a gearing mechanism is released quickly for improved downshift feel.